Fix gcc warning -Wsign-compare

Add the compile flag '-Wsign-compare' and adapt the code to fix all
cases of this warning.

Change-Id: I26b08fa5a03e4ac294daf697d32cf9140d84350d

meson.build

@@ -485,6 +485,7 @@ add_project_arguments(
   # TODO(Qyriad): Yes this is how the autoconf+Make system did it.
   # It would be nice for our headers to be idempotent instead.
   '-include', 'config.h',
+  '-Wsign-compare',
   '-Wno-deprecated-declarations',
   '-Wimplicit-fallthrough',
   '-Werror=switch',

src/libstore/build/worker.cc

@@ -247,7 +247,7 @@ Worker::Results Worker::run(std::function<Targets (GoalFactory &)> req)
                        .exclusiveJoin(std::move(onInterrupt.promise));
 
     // TODO GC interface?
-    if (auto localStore = dynamic_cast<LocalStore *>(&store); localStore && settings.minFree != 0) {
+    if (auto localStore = dynamic_cast<LocalStore *>(&store); localStore && settings.minFree != 0u) {
         // Periodically wake up to see if we need to run the garbage collector.
         promise = promise.exclusiveJoin(boopGC(*localStore));
     }

tests/unit/libutil/compression.cc

@@ -147,7 +147,7 @@ TEST_P(PerTypeNonNullCompressionTest, truncatedValidInput)
     /* n.b. This also tests zero-length input, which is also invalid.
      * As of the writing of this comment, it returns empty output, but is
      * allowed to throw a compression error instead. */
-    for (int i = 0; i < compressed.length(); ++i) {
+    for (int i = 0; i < (int) compressed.length(); ++i) {
         auto newCompressed = compressed.substr(compressed.length() - i);
         try {
             decompress(method, newCompressed);